Jailbreak

Cydia

So if you haven't already discovered tethering is working on iPhone firmware 3.0 (even for AT&T). You can enable AT&T tethering via a simple .ipcc (download AT&T.ipcc) update via iTunes OR you can make it happen in an even easier manner via Safari on your iPhone!

BenM.at is hosting TONS of iPhone tethering configuration files for many many networks across various countries.